1. Technical Field of the Invention
The present invention relates generally to a gas concentration measuring apparatus which may be used in measuring the concentration of a preselected component of exhaust emissions of automotive engines, and more particularly to such a gas concentration measuring apparatus equipped with a failure monitor designed to detect a failure in operation of a gas sensor such as a wire disconnection, a power supply short, or a ground short.
2. Background Art
Limiting current air-fuel (A/F) ratio sensors (also called lambda sensors) are known which measure the concentration of oxygen (O2) contained in exhaust emissions of motor vehicle engines as indicating an air-fuel ratio of a mixture supplied to the engine. A typical one of the A/F sensors includes a sensor element made up of a solid electrolyte body and a pair of electrodes affixed to the solid electrolyte body. The measurement of concentration of oxygen is achieved by applying the voltage to the solid electrolyte body through the electrodes to produce a flow of electrical current through the sensor element as a function of the concentration of oxygen and sampling the electrical current to determine the A/F ratio.
Usually, if terminals connecting with ends of the sensor element are disconnected, short-circuited to a power supply, ground, or each other, it will result in a failure in measuring the current flowing through the sensor element accurately, which leads to an error in determining the concentration of oxygen contained in the exhaust emissions of the engine (i.e. the A/F ratio of a mixture supplied to the engine). In order to avoid this problem, conventional systems are designed to sample and take voltages appearing at the terminals of the sensor element into a CPU through A/D converters and determine that the A/F sensor is malfunctioning when outputs of the A/D converters are shifted out of a normal voltage range. This structure requires as many additional A/D converters as the terminals of the sensor element, thus resulting in increased complexity of the structure. Improvement of such a structure is, therefore, being sought.
Japanese Patent No. 3446400, assigned to the same assignee as that of this application, teaches a failure diagnosis system for A/F sensors used in air-fuel ratio control of automotive engines which is designed to monitor the degree of activation of the A/F sensor to determine whether the A/F sensor is malfunctioning or not. The failure diagnosis system also works to determine that the A/F sensor is malfunctioning if the A/F sensor is determined not to be activated yet after once being determined to have been activated after start-up of the engine. Many other failure diagnosis systems for A/F sensors have been proposed in recent years, but however, emission regulations have been increasingly tightened. The failure diagnosis systems are, therefore, being required to detect the failure of the A/F sensor with high accuracy and/or identify the cause thereof.